In recent years, in the field of the disk storage apparatus such as a hard disk drive (hereinafter, abbreviated simply as a disk drive), higher recording densities of a disk as a storage medium are promoted. In the disk drive, servo areas (also called a servo frames) are arranged at fixed intervals in a circumferential direction on the disk. The each servo area is a recording area of servo information. The servo information is position information to detect the position of a head over the disk. The servo information includes a cylinder number (track number) to identify the position of a cylinder (track) and a sector number to identify a sector in the cylinder.
Incidentally, a servo information division method of dividing and recording cylinder information constituting the cylinder number to identify one cylinder in a plurality of servo areas in a cylinder is proposed in order to implement high-recording density of a disk. Similarly, a method of dividing and recording the sector number in the plurality of servo areas is proposed. Because the size of each servo area can be reduced by these methods, a data area where user data is recorded for each cylinder can, as a result, be increased.
According to the servo information division method, because cylinder information is divided into a plurality of servo frames, the current position or current velocity during seeks cannot be correctly obtained when seeks are done from the current position to a target position. Thus, seek control using a combined position from a predicted position by a state observer as the current position and obtained cylinder information is performed. In this case, a state offset as an error between the combined position and a predicted velocity by the state observer, and an actual position and an actual velocity of the head may respectively occur due to factors such as a disturbance. The seek control is performed using the position. In this case, a state offset as an error between an estimated position of the state observer and the actual position of the head based on servo information may occur due to factors such as a disturbance. During such a state offset occurred, the precision of seek control is degraded so that the head may not be able to reach the target position.